OK, we did this awhile back, but I can't remember the thread, or find it...

The OP of that thread wanted to do the same, I advised him to put PRs in the sides of the sealed chamber instead.

He did it with one PR, and found that there was a percieved (no meters) increase in lower extension, as well as a percieved decrease in SQ, though he himself admitted that there was no real way to tell the difference between yucky sounds from the box and his house rattling.

In the end he removed the PR from the sealed chamber and ran his HWK stock. Thats the story, and IMHO, he didnt have enough Sd for PR's to do the work he wanted, for that setup it would have been either two very heavily damped PRs or four normal ones in the sealed chamber. A general rule of thumb is 2x Sd at the minimum for good control and gain.

At that time I had advised him against putting a PR inside the box, in place of a driver, stating that a PR in one side would be like feeding the port for that side with another port. In theory this works, a la Graham Hollimann infra-bass resonator (have white papers etc, PM me), however the side you drive with the PR HAS to be tuned VERY low, as driving a chamber tuned to 30Hz with a PR that resonates at 13Hz (on a graph a PR makes gain the same way a port does, only with no port noise, and the ability to simulate a MUCH longer port) can be umm, well its almost impossible, see the outside port would only make noise when the acoustic impedance of the enclosure dropped below the level required to limit pressure. At this point the port would indeed resonate, at a very low frequency and with a fair amount of gain, the math is way over my head, but if you wanted decent output from the PR you would need to tune the chamber it was adjacent to to the frequency where the PR actually makes gain (the heavier the cone, the lower this goes, making for ports that can get unmanageable). Imagine trying to tune the 3.5 cubes or so available in the HWK15 to lower than 20Hz! (a peerless XLS 10" PR with only 200g of mass added resonates between 13 and 16 hz!)

It is, of course, completely up to you, I can send you what I have on the infra-bass resonator (the closest thing, I know of, to what you describe), and you can try to graft it onto a HWK. I cant see it working all that well though, as the powered cone would be fighting the acoustic impedance of its adjacent chamber, the weight of the PR and airspring, as well as the acoustic impedance of the other chamber, not to mention the forces generated by the PR once it is in motion.

A safe compromise would be to put in two working drivers, and leave one unhooked, this would double the Mms, yeilding a modified Qts that "might" be favorable, it would also affect Vas, making for a better tuning overall (similar to an iso-load in that the cones would still cancel each others irregularities, though the apparent volume might not actually double as if there were two working motors in there) In this setup you dont run the risk of the PR losing control (and running away with all your drivers Xmech) because the passive cone is the same weight/compliance/Fs as the powered one making it much easier to control. I will try it in my HWK8 this weekend if you like and see what happens to the response, but being as I have no meters, it will be a subjective review.

Now, replacing the port on the low tuned side with a set of PRs, and running either dual or quad loaded drivers inside could do away with a LOT of port noise issues that are prevalent in the low side of this design, though I cant say how that setup would affect the response on the high tuned side, especially given that the PRs would push back on the high tuned side when they got excited.